pdftables: Programmatic Conversion of PDF Tables

Allows the user to convert PDF tables to formats more amenable to
analysis ('.csv', '.xml', or '.xlsx') by wrapping the PDFTables API.
In order to use the package, the user needs to sign up for an API account
on the PDFTables website (<https://pdftables.com/pdf-to-excel-api>).
The package works by taking a PDF file as input, uploading it to PDFTables,
and returning a file with the extracted data.